1. Field of the Invention
The present invention relates to an improved process for the production of polysaccharides by the fermentation of carbohydrates utilizing microorganisms. More especially, this invention relates to a fermentation process using a starch or a hydrolysate of a starch as the source of nutritive carbon.
2. Description of the Prior Art
Fermentation polysaccharides having high molecular weights, or biopolymers, are increasingly in demand for numerous industrial applications by reason of their thickening, viscosifying and stabilizing properties in aqueous media. Thus, xanthan gum, in light of its exceptional rheological properties, is used in fields as varied as the building, paint, paper, textile, cosmetic and food industries, in agriculture, water treatment, drilling, petroleum recovery, and many others.
Biopolymers are products of aerobic cultures of microorganisms in an aqueous nutritive medium.
Xanthan gum is produced by bacteria of the species Xanthomonas. Biopolymers of the same type may be produced by a wide variety of microorganisms, including, among the best known, those of the species Agrobacterium, Arthrobacter, Alcaligenes (Succinoglycane), Pseudomona (Levan), Rhizobium, Sclerotium (Scleroglucane). These polysaccharides have high molecular weights, typically higher than 1.times.10.sup.6 and consist of glucose, mannose, galactose, rhamnose, glucuronic acid, mannuronic acid, guluronic acid units, and possibly the acetate and pyruvate derivatives thereof. Their particular structures and properties are described, for example, in Whistler, Industrial Gums, 2nd Edition, Chapters XXI-XXIII (1973).
Numerous publications exist relating to the production of fermentation polysaccharides. Processes for the production of xanthan gum are described, for example, in U.S. Pat. Nos. 3,020,206, 3,251,749, 3,391,060, 3,271,267, 3,427,226, 3,433,708, 3,455,786, 3,485,719, 3,594,280, 4,154,654 and 4,282,321.
The nutrient medium normally contains, in addition to different growth elements, assimilable carbohydrates as the source of carbon. Suitable carbohydrates include glucose, saccharose, fructose, maltose, lactose, soluble starches and their hydrolysates. Although crude (raw) starch is described as a suitable source of carbon, it presents the major disadvantage of considerably extending the fermentation cycle relative to the monosaccharides, such as glucose. Furthermore, the microorganism is not capable of consuming all of the reducing sugars. The presence of these residual sugars at the end of the fermentation, on the one hand, renders the medium susceptible to the development of contaminating strains capable of degrading the wort prior to the separation of the polysaccharide. On the other hand, it may adversely color the product during the heat treatments of pasteurization and optional clarification.